Low cost imaging arrays fabricated in CMOS are utilized in numerous cameras. In digital photography, the image is recorded by an imaging array in which each pixel includes a photodetector that measures the amount of light that falls on some portion of the pixel area. Imaging arrays that have a high dynamic range are required for many applications, including scientific research photography. The dynamic range of an imaging array will be defined to be the ratio of the maximum signal for a pixel to the minimum signal that is above the noise.
Typically, the imaging arrays are constructed from a two-dimensional array of pixel sensors organized as a plurality of rows and columns of pixel sensors. All of the pixel sensors in a given column are connected to a common signal line. The specific pixel in a column that is currently being readout is determined by a row select signal that connects all of the pixel sensors in a given row to their respective signal lines.
To improve the readout time of the array, each signal line has a dedicated signal amplifier and readout circuit so that the entire row of pixel sensors can be readout in parallel. The readout amplifier and associated analog-to-digital converters (ADCs) must have a dynamic range that is commensurate with the dynamic range of the individual pixels and must provide a readout time for each pixel that is as short as possible, particularly in low light images, as the noise in each pixel value increases with the time between the end of an exposure and the time the pixel is readout. To increase the dynamic range of the readout circuit, schemes in which multiple readout amplifiers are used on each signal line have been proposed. For example, U.S. patent application Ser. No. 12/036,998 filed Feb. 25, 2008 describes a readout system in which digitization circuits having different gains are implemented for each readout line, the choice of digitization gain being determined by the signal on the readout line in question.
While multiple readout digitization circuits can provide improved dynamic range, the improvement comes with a significant cost, since the number of transistors that must be included in the processing circuitry for each signal line is substantially increased. Since there can be thousands of columns of pixels, hence, signal lines, each additional transistor represents a significant increase in the area of silicon required to implement the readout array, and hence, the cost of the image sensor.